1. Field of the Invention
The field of the invention relates to integrated fiber optic based dynamic light scattering apparatus including an electronic processing system for the characterization of the size of the submicroscopic scatterers, including colloidal suspensions, polymers, and physiological structures such as proteins, a method of determining the size of such particles, and probes for detecting scattered light.
2. Brief Description of the Prior Art
Dynamic light scattering provides a temporal characterization of the scattering species executing Brownian motion; the scattering species are usually in the size range 3 nm to 3000 nm and can be colloidal particles, proteins, polymers or random phase fluctuations caused by density or temperature fluctuations. The technique requires coherent illumination and coherent detection from the scattering species. The accepted practice is to detect the scattered light as a series of photon events, which are processed using a real time digital correlator to yield an intensity autocorrelation. The intensity autocorrelation data, using suitable modeling, is inverted to yield a distribution of sizes or molecular weights. Correlation techniques have become the main stay of the state of art systems in use today. U.S. Pat. Nos. 4,975,237 and 4,983,040 disclose systems using correlation techniques for analyzing light scattered by particles undergoing Brownian motion. The photons streams may, however, be processed using equivalent spectral estimation techniques. Both techniques require several minutes of data acquisition followed by several minutes of data analysis prior to displaying the distribution of sizes.
The known systems for determining the mean diameter of particles exhibiting Brownian motion are relatively expensive, and require hardware which may be too large in size for certain field uses. Digital correlation which is the most often used approach for the experimental investigation of stochastic systems, requires an expensive piece of hardware in the correlator. Systems including such correlators further require computers having sufficient computing power to perform the necessary mathematical calculations. While the use of a PC or laptop computer in a lab or other convenient setting is not a problem, transporting such equipment to more inhospitable areas can be a significant drawback.
Probes employed for illuminating particles and/or detecting light scattered thereby have included several designs. In accordance with one probe embodiment shown in U.S. Pat. No. 5,155,549, both the transmitting and receiving fibers are coupled to a gradient index microlens. Such a microlens greatly increases the size of the probe, which may not be acceptable in certain circumstances. Alternative structures disclosed in the patent include a lensless probe, and one in which a microlens is formed on the end of the transmitting fiber either by dipping the fiber or by heating the fiber end such that the core thereof diffuses into the cladding. An integrated fiber optic probe used as a receiver of scattered light is disclosed in an article by Dhadwal et al., "Integrated fiber optic probe for dynamic light scattering", Applied Optics 32, 3901-3904 (1993). The probe includes a monomode fiber fusion spliced to a short length of graded-index multimode fiber. An unfocused, collimated laser beam was used to provide light to the sample cell.